1. Field of the Invention
The present invention relates to a method for manufacturing products by bonding together the powder in a shaped object to form a dense body by means of isostatic hot pressing of the powder under high pressure, (pressure sintering). During the sintering the powder billet is enclosed in a metal container which is placed in a pressure furnace and is subjected in this furnace simultaneously to a high, all-sided gas pressure and high temperature. As a rule, the object formed by the powder is degassed during the first part of the heating prior to the sintering by means of vacuum suction in the container, in order to remove gases which have combined with the powder or been bound to the surface of the powder grains. As a rule, this degassing takes place at very low pressure by connecting the container enclosing the powder body to a vacuum pump. The invention is of particular interest in the manufacture of billets, for example billets of material of high-speed steel type or so-called superalloys, said billets being further processed to the desired shape by rolling, forging or machining.
2. The Prior Art
Powder Metallurgy 1964 Vol. 7 No. 14, pages 168- 201 , contains a detailed article on pressure sintering by E.S. Hodge, Interceram No. 4 1967 and Nos. 1,2, and 3 1978 contain a detailed description of Isostatic pressing by E.L.J. Papen. The invention can be applied in the method described in, for example, Lundstrom application Ser. No. 866,885, filed Oct. 16, 1969, of which a continuation-in-part application, Ser. No. 68,665 was filed on Sept. 1, 1970. The equipment described therein may be used.
The method according to the invention relates to a way of connecting a suction line to a metal container which encloses a powder body, and the design of the connection of an evacuation tube to the container.
Metal powder which is brought into contact with air absorb gases present in the air on its surface and forms compounds with these, especially oxides. These absorbed gases and oxides are removed to the greatest possible extent since the powder is degassed during the heating prior to the sintering. In the degassing of powder bodies which are enclosed in a metal container, the container, mostly a lid, has been provided with a welded-on tube which is connected to a vacuum pump during the degassing process. However, it has proved very difficult to arrange a large evacuation opening in the container which gives little resistance to evacuation of gases, and at the same time achieve a connection between the container and a welded-on tube which withstands the following isostatic pressure sintering. The weakest spot is the welded joint between the container and the tubular element, by means of which the container is connected to the vacuum pump. This welded joint has burst when affected by the gas pressure acting on the container during the sintering. The difficulties in achieving a solid welded joint have increased with the increased size of the evacuation opening and the resultant increased diameter of the connection tube welded round the opening.